Although used extensively in integrated circuits, CMOS devices are not ideal switches for logic functions because there exists a small leakage current. Various forms of leakage current are shown in FIG. 1a–3b. FIGS. 1a and 1b illustrate the generation of “sub-threshold” leakage current between the source and drain nodes that exists even when the transistor is in the “off” state when the voltage between gate and source node (VGS) is lower than a threshold voltage (VT) of the transistor, i.e., when |VGS|<|VT|. Generally, the sub-threshold current depends strongly on the voltage difference between drain and source nodes (VDS), the threshold voltage (VT) and temperature.
Another leakage source also exists, as shown in FIGS. 2a and 2b, between the drain/source nodes and the bulk of a MOS device. This current, referred to as a junction leakage current, strongly depends on the source-bulk voltage (VSB) and the drain-bulk voltage (VDB), the source/drain implant and the junction temperature.
A third leakage current, as shown in FIGS. 3a and 3b, is gate node leakage current that exists between the gate to bulk node and the gate to source/drain nodes. This gate node leakage current, which is referred to as gate leakage current, depends on the cross voltage between gate and bulk nodes (VGB), the cross voltage between gate and source/drain nodes (VGS/VGD) and the gate oxide thickness. These leakages may exist singularly or, more typically, concurrently in a MOS transistor and effect the performance of the devices fabricated from the MOS transistors.
Hence, there is a need in the industry for a current monitoring means that can measure these leakage currents individually or in combination.